Picture this: In the Old West, three lawmen are in the desert when they happen upon an abandoned wheelchair in the sand.

Says a member of the posse: “Don’t worry, he won’t get far on foot.”

That was just one of hundreds of razor-sharp, politically incorrect, hilarious and often offensive cartoons to spring from the mind of John Callahan. It was the title of Callahan’s 1989 autobiography, and now it’s a Gus Van Sant movie, with Joaquin Phoenix disappearing into the role of the hard-drinking free spirit who was paralyzed in a car accident after a wild night of partying, battled alcoholism for most of his life — and produced some of the most memorable cartoons of the late 20th century.

The great Van Sant is one of the most versatile directors in the world, capable of delivering edgy, avant-garde work such as “Drugstore Cowboy,” “Gerry” and “Elephant,” as well as crowd-pleasing mainstream fare such as “Good Will Hunting” and “Finding Forrester.”

Given the subject matter of “Don’t Worry,” I assumed this would be one of Van Sant’s more experimental adventures, filled with innovative animation, varying photography styles and perhaps a teardown of the fourth wall here and there.

I couldn’t have been more wrong. Writer-director Van Sant has delivered a conventional biopic, with the primary focus on Callahan’s climb up the 12-step ladder of recovery. There are a few clever touches — e.g., occasionally we see one of Callahan’s drawings come to life as a brief bit of animation — but “Don’t Worry” is essentially a warm-hearted, sometimes almost gooey story of a man who suffers greatly before learning to look outside himself and realize the richness of the life he’s been given.

With the exception of a few flashbacks and early scenes, Phoenix spends the bulk of the film in a wheelchair, his limbs contorted, swearing in frustration when he’s unable to open a bottle of wine by himself, struggling to hold a pen as he painstakingly sketches out his simple but beautiful drawings.

It’s hardly a surprise Phoenix is so good — he’s one of our best chameleons — but it IS a bit of a surprise that he never launches into anything approaching Method madness, not even when he’s talking to a vision of his birth mother or getting blackout drunk. It’s a performance to suit the material.

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Abandoned by his mother at birth, Callahan started drinking heavily when he was in his early teens. “Don’t Worry” kicks off Callahan’s story in 1972, when he was 21 years old and all he cared about was getting ripped. After an intense night of partying with Dexter, a guy he’d just met, Callahan climbs into the passenger seat of his car — and wakes up in the hospital, unable to move, after Dexter slammed the car into a light pole while going 90 mph. Jack Black plays Dexter. He’s in the movie for maybe 10 minutes, total, and yet it’s a performance you won’t forget.

And so begins Callahan’s long, sometimes torturous journey to sobriety and self-actualization.

Playing a composite character, Rooney Mara affects a Swedish accent and is back-lit like an angel as Annu, a physical therapist and stewardess (as they were known back then) who falls in love with Callahan.

It’s even more over-the-top than it sounds. I kept waiting for Callahan to dictate a letter: “Dear Penthouse: I never thought anything like this could happen to me, but it’s all true …”

The film sings with creativity and a feel-good ’70s vibe and some fantastic dark humor in the scenes featuring a marvelous performance by Jonah Hill as an openly gay, Gregg Allman-looking, trust-fund baby who conducts unorthodox recovery meetings in his lavish and enormous mansion.

The regulars in the group include Kim Gordon from Sonic Youth as a former suburban housewife; the offbeat German character actor Udo Kier as Hans, who delivers some appropriately weird lines, and the singer-songwriter Beth Ditto, who is flat-out amazing as Reba, a plus-sized gal who cuts Callahan to the quick with a scathing indictment of his pity-party approach to life.

We’re nearly halfway through the movie before Callahan tries his hand at panel cartoonist work, and even after that we see only brief glimpses of his dark humor, which delighted many and offended quite a few.

Another example of Callahan’s humor: A man in sunglasses stands on a street corner, holding out a cup and wearing a sign saying, “Please help me. I am blind and black but not musical.”

If you don’t know Callahan’s work, please take a few minutes to look up some of his cartoons. My guess is you’ll find some of the stuff dated and filled with narrow-minded stereotypes.

And you’ll find plenty that will make you smile or even laugh out loud.

‘Don’t Worry, He Won’t Get Far on Foot’

★★★

Amazon Studios presents a film written and directed by Gus Van Sant, based on the memoir by John Callahan. Rated R (for language throughout, sexual content, some nudity and alcohol abuse). Running time: 113 minutes. Opens Friday at local theaters.